The present invention relates to a thermochromic coating and a process for the production thereof. Coatings of this type are intended for use in building construction for glass windows or glass cladding in order to be able to influence the room climate in the interior of the building by utilizing the transmission factor of these coatings, which changes with temperature.
A coating material which has proven advantageous for the coating of architectural glass is vanadium oxide. It is known that crystalline vanadium oxide has a switching temperature Ts in the region of 68xc2x0 C. I.e. a semiconductor-metal phase transition occurs at this temperature, so that the transmission factor, in particular in the infrared region, i.e. in the wave-length range of 1000 nm and above, is significantly lower at temperatures above this switching temperature Ts owing to the metallic state which then exists, than at a temperature below the switching temperature Ts, at which the vanadium oxide, i.e. vanadium dioxide, is in the semiconductor state.
In the past, various attempts have been made to shift this switching temperature in the region of 68xc2x0 C. into the region of room temperature. It has been found that doping of the vanadium oxide with tungsten results in a shift in the switching temperature into the region of room temperature, where the tungsten content here should be in the region of 2.6%. However, it has furthermore been found that the incorporation of tungsten can result in a slight reduction in the transmission factor in the visible region.
It is furthermore known to dope vanadium dioxide layers with fluorine, where, in the case of fluorine doping, the problem exists that on deposition of these layers by reactive high-frequency or radiofrequency sputtering, incorporation of fluorine into the vanadium oxide lattice is only obtained at deposition temperatures of below 600xc2x0 C. However, such low deposition temperatures have an effect on the crystal structure of the vanadium oxide lattice, with the result that a transition from the polycrystalline to the amorphous state can occur with falling deposition temperature. The consequence of this increasing disorder in the lattice is an impaired switching property of the layers obtained in this way.
It is an object of the present invention to provide a thermochromic coating, in particular for glass, which, while having good switching properties, has a switching temperature in the region of room temperature and a transmission behavior in the visible spectral region which is improved compared with known layers. It is a further object of the present invention to provide a process for the production of layers of this type.
The first-mentioned aspect of the object is achieved in accordance with the present invention by a thermochromic coating, in particular for glass, which comprises a vanadium oxide layer which furthermore comprises tungsten and fluorine.
It has been found that co-doping of the vanadium oxide lattice with tungsten and fluorine gives rise to a synergistic effect which was not to be expected from the two above-outlined individual behaviors of tungsten-doped coatings on the one hand and fluorine-doped coatings on the other hand. Co-doping or vanadium oxide with tungsten and fluorine firstly has the consequence that, owing to the incorporation of tungsten, the switching temperature Ts is lowered into the region of room temperature. Furthermore, the simultaneous incorporation of fluorine has the consequence that the transmission factor of the coatings produced in this way in the visible and ultraviolet spectral region, i.e. in particular in the wavelength range of 500 nm or less, is improved. The consequence of this is that the outer appearance can be significantly improved owing to the improved color neutrality compared with known thermochromic coatings, for example vanadium oxide layers doped merely with tungsten, through the increase in the transmission factor in the wavelength range mentioned. These known coatings generally have a color effect shimmering yellow and green.
The tungsten content in the thermochromic coatings according to the present invention can be in the range 2.6 atom-%.
In a corresponding manner, the fluorine content can be in the range from 0.01 to 2 atom-%, preferably from 0.5 to 1.5 atom-%.
In order to be able to avoid the above-mentioned problem of the increasing disorder in the lattice on reduction of the room temperature, which is necessary for the incorporation of fluorine, on deposition of the layers according to the invention, it is furthermore proposed that an intermediate layer be provided between the vanadium oxide layer and a substrate carrying this vanadium oxide layer, where the intermediate layer comprises a titanium oxide layer. It has been found that, if the vanadium oxide layer to be doped with fluorine and tungsten is deposited on this titanium oxide layer, the stresses present in the vanadium oxide lattice can be reduced and the growth conditions for the vanadium oxide layer at low deposition temperatures, for example in the region of 300xc2x0 C., can be improved, so that the reduction of the deposition temperature to the stated range does not result in a corresponding impairment of the optical properties. The invention furthermore relates to a thermochromic coating, in particular for glass, comprising a vanadium oxide layer which comprises tungsen, preferably in the range from 1.0 to 2.6 atom-%, most preferably in the range from 1.6 to 2 atom-%, and where the coating furthermore comprises an intermediate layer of titanium oxide which is or can be arranged between the vanadium oxide layer and a substrate carrying the vanadium oxide layer. It has been found that the provision of the titanium oxide intermediate layer may also be advantageous in the case of the use of vanadium oxide doped merely with tungsten, in particular if, for example, the deposition temperature is lowered in order to influence the tungsten content in the thermochromic layer. Owing to the titanium oxide layer, the deposition conditions for the tungsten-doped vanadium oxide layer are improved, as mentioned above, so that the optical properties of thermochromic coatings of this type are improved in a corresponding manner.
The invention furthermore relates to a thermochromic coating, in particular for glass, comprising a vanadium oxide layer which comprises fluorine, preferably in the range from 0.01 to 3 atom-%, most preferably in the range from 0.5 to 2.5 atom-%, and where the coating furthermore comprises an intermediate layer of titanium oxide which is or can be arranged between the vanadium oxide layer and a substrate carrying the vanadium oxide layer. The above-described advantageous effect of a titanium oxide intermediate layer can also result in considerable improvements in the optical properties in the case of a vanadium oxide layer doped merely with fluorine.
In order to prevent post-oxidation of the vanadium oxide layerxe2x80x94doped with tungsten or tungsten and fluorinexe2x80x94it is proposed that a silicon oxynitride cover layer furthermore be provided on the vanadium oxide layer. This silicon oxynitride layer results not only in the prevention of post-oxidation, but also forms, in particular, an antireflection layer, through which the transmission, in particular in the visible spectral region, can additionally be improved.
It has proven advantageous or the cover layer to have a thickness in the range from 10 to 300 nm, preferably from 10 to 100 nm.
The vanadium oxide layer can have a thickness in the range from 30 to 350 nm, preferably from 50 to 150 nm.
The intermediate layer mentioned can have, for example, a thickness in the range from 10 to 100 nm, preferably from 30 to 70 nm.
The second-mentioned aspect of the object indicated above is achieved in accordance with the present invention by a process for the production of a thermochromic coating, in particular for glass, which comprises a) the deposition of a vanadium oxide layer doped with tungsten and fluorine.
The deposition of the vanadium oxide layer can be carried out by a sputtering process, preferably high-frequency sputtering or magnetron sputtering.
This vanadium oxide layer doped with tungsten and fluorine can be deposited, for example, by reactive high-frequency or radiofrequency sputtering of a tungsten-containing vanadium target in an atmosphere comprising an oxygen/argon mixture and trifluoromethane (CHF3).
The trifluoromethane partial pressure here is preferably in the range 0.5-5xc2x710xe2x88x923 Pa.
Furthermore, the deposition power is preferably in the range 200-600 W and the deposition temperature is preferably in the range 200-600xc2x0 C., most preferably about 300xc2x0 C. This is a temperature range in which the incorporation of fluorine into the vanadium oxide lattice is also achieved.
In order to prevent an amorphous vanadium oxide phase mixture, which does not exhibit a metal-semiconductor phase transition in the range below 100xc2x0 C., from being generated during the deposition, a titanium oxide layer is preferably deposited on the substrate before step a). As already mentioned, this titanium oxide layer has the consequence that the stresses generated in the vanadium oxide lattice are reduced and improved growth conditions are created for the vanadium oxide layer, in particular at deposition temperatures in the region of 300xc2x0 C.
The titanium oxide layer can also be deposited by a sputtering process, preferably magnetron sputtering or reactive high-frequency or radiofrequency sputtering, of a titanium target in an atmosphere comprising an oxygen/argon mixture.
When carrying out reactive high-frequency sputtering, the deposition power during deposition of the titanium oxide layer is preferably in the range from 200 to 600 W and a deposition temperature during the deposition of the titanium oxide layer is preferably in the range from room temperature to 600xc2x0 C.
In order to prevent post-oxidation of the deposited vanadium oxide layer and impairment of the optical properties induced thereby, it is proposed that a silicon oxynitride layer be deposited on the vanadium oxide layer after step a). As already mentioned, this silicon oxynitride layer furthermore has a consequence of the advantageous effect that it acts as an anti-reflection layer, and the transmissivity of the coating according to the invention, in particular in the visible spectral region, is thereby significantly improved.
The silicon oxynitride layer can be deposited, for example, by a sputtering process, preferably magnetron sputtering or reactive high-frequency or radiofrequency sputtering, of a silicon target in an atmosphere comprising an argon/oxygen/nitrogen mixture.
When carrying out reactive high-frequency sputtering here, the deposition power is preferably in the range from 200 to 300 W and the deposition temperature during deposition of the silicon oxynitride layer is preferably in the range from room temperature to 200xc2x0 C.
The present invention furthermore relates to a process for the production of a thermochromic coating, in particular for glass, comprising:
a) deposition of a titanium oxide layer on a substrate, preferably a glass substrate, and
b) deposition of a vanadium oxide layer, preferably doped with tungsten or fluorine, onto the titanium oxide layer.